1. Field of the Invention
The present invention relates to an improved method for producing white liquor by a combined slaking and causticizing operation.
2. Description of the Prior Art
In conventional methods for alkaline pulping of cellulosic materials, the slaking of the lime and subsequent causticization of the green liquor is accomplished in separate units, each specially designed to carry out a single step of the process. Thus, strong green liquor is fed to a lime slaker-classifier unit where it is mixed with and slakes a measured flow of lime. The resultant mix of lime flows to a classifier section for removal of coarse grit and unreacted material by means of a conveyor or rake-like device moving up the inclined bottom of the classifier tank. The degritted slurry overflows from the slaker-classifier into a series of tanks in which it is agitated and the causticizing reaction, already started in the slaker, is completed. The causticized slurry then passes to a clarifier where the precipitated lime mud is separated out and is usually pumped to a thickener for recovery. The strong white liquor removed from the clarifier is then ready for use in cooking.
One serious disadvantage of the conventional slaking-causticizing system is the tendency for portions of the reacting slurry to overflow from one stage to the next without being retained in a particular reaction zone for the full time intended. This short-circuiting or bypassing of relatively unreacted chemicals is relatively inefficient and does not utilize the available chemicals to the best advantage.
Various combined pressurized slaker/causticizing systems have been proposed; however, none have proven sufficiently cost-effective or efficient to replace the conventional two-stage systems.
One such combined system employs a slaker with an air lock to remove grits. The system, however, does not satisfactorily resolve the problem of grits removal.
Another system utilizes a system wherein the reactor is maintained under pressure, but wherein the slaker and causticizer are unpressurized. These units operate at very low efficiency and have not found widespread acceptance in the industry.
U.S. Pat. No. 2,539,732 discloses a process for slaking and causticizing lime in a single unit. In the causticizer unit, lime (CaO) is introduced concurrently with green liquor. The lime is slaked and then gently mixed with the green liquor in a series of reaction trays to complete the causticizing. The unit can be used in cases where the lime (solid) is in suspension in a liquid (green liquor) before introduction into the unit. The unit is provided with a plurality of trays and scraper/agitator means for moving solid material from one tray to the next lower tray. The patented system does not provide, however, for an efficient removal of grits and other solids from the various product and intermediate liquors.
U.S. Pat. No. 3,194,638 relates to a combined slaker/causticizer. Again, the problem of efficient grit removal is not adequately addressed.
U.S. Pat. No. 4,627,888 discloses high pressure slaking followed by atmospheric pressure causticizing, whereas U.S. Pat. No. 4,762,590 discloses a process wherein both the slaking and causticizing can be performed at elevated pressure. However, the latter patent discloses slaking lime with white liquor rather than with green liquor.
The present invention provides a single-stage slaking/causticizing system which provides for an essentially one-step slaking and causticizing operation for the efficient and cost-effective production of white liquor which drastically reduces the number of separate apparatuses required in conventional systems and which provides for the complete removal of grits and other contaminating solids from the white liquor.